UKH

There seems to be broad agreement that maintaining the right level of hydration is critical, not just to peak athletic performance, but life itself; too little water or too much and you die.

Advice from the NHS website is that we should be drinking 8 glasses of water per day.

I drink around half that. 
Why am I not dead?

Dunno. I drink 7 glasses per day, and I am dead.

I drink zero glasses of water, I do however drink many cups of tea.

What size glass?  I believe the 8 glasses came from a time when drinking glasses were rather smaller. 

Edit, seems it is a bit of a myth.  

https://healthblog.uofmhealth.org/womens-health/myth-busting-no-more-8-glas...

I have water with lunch and lots of tea.  If the urine colour chart is correct I'm plenty hydrated.  

https://dripdrop.com/blogs/news/chart-dehydrated

Humans managed to survive for millions of years with out NHS advice. I dunno, maybe just drinking when you feel the need actually works?  

A lot of authorities on the subject concede the fact that beer is just another form of water.

( Seriously)

Absolutely, we have a finely tuned mechanism called 'thirst' to let us know when we should drink more.  The 8 glasses thing has no scientific basis - it originally came from a calculation of total fluid intake from food and drink.

references..

https://www.nytimes.com/2015/08/25/upshot/no-you-do-not-have-to-drink-8-gla...

Ditto, "Well hydrated", which is like "well cool", possibly

Ditto, "Well hydrated", which is like "well cool", possibly

The food you eat can also count towards your 8 glasses.

I was told by an ex army medic that your thirst response is a bit slow and by the time it kicks in your already a dehydrated.    

thirst isn't quite such an aid to knowing when to drink for older people.  Dehydration is not an uncommon problem

Why do you think there are/were so many pubs around.  It wasn't that long ago that drinking water was a hazardous operation and for adults hydration was via very weak beer.

To Imhambane - I have also heard that thing about thirst response being slow, but also that it's an old wives tale and urine colour is your guide

I run ultra distance races so take a keen interest in taking in the right amount of fluid during races (where it's not unusual to lose a couple of KGs during a race, most of it water) and the golden rule there too is that if you're thirsty, you're already dehydrated.

Of course the opposite problem, more common in shorter events like marathons, is when people take in too much fluid which can bring on hyponatremia, where you're essentially diluting the sodium in your bloodstream.

Well that's good news.😁